1. Field of the Invention
This invention pertains to a system for transmitting a signaling cell between a user terminal and a signaling terminator of an ATM (Asynchronous Transfer Mode) switch, and more particularly to a signaling cell switching system for variably setting a signaling cell's transmission path and a method of doing same.
2. Description of the Related Art
ISDNs (Integrated Services Digital Networks) are being developed as alternatives to conventional communication networks. A wide-band ATM exchange system stores split communication information in fixed-length cells. These cells are transmitted over an optical transmission path utilizing high-speed switching hardware, enabling a large amount of information to be transmitted efficiently. As in conventional digital switching, an ATM switch requires a sequence for setting a switching path from a sending terminal to a receiving terminal at a call setting time. Therefore, it is necessary to transmit signaling information output from a sending terminal via a network in the ATM switch to a signaling terminal unit at a call setting time.
FIG. 1 shows an example of a configuration of an ATM switch. In FIG. 1, 10.sub.00 through 10.sub.m0 and 10.sub.01 through 10.sub.m1 are VCI (Virtual Channel Identifier) converters (VCC), 4.sub.00 through 4.sub.20 and 4.sub.01 through 4.sub.21 are ATM switching units, 6.sub.0 and 6.sub.1 are signaling terminals (SIG), and 17 is a switching controller. ATM switching units 4.sub.00 through 4.sub.20 and 4.sub.01 through 4.sub.21 perform an (m+1) * (m+1) switching. HW.sub.00 through HW.sub.m0 and HW.sub.01 through HW.sub.m1 attached to the input side and the output side of respective ATM switching units are input ports and output ports for data highways. As described above, the ATM switch shown in FIG. 1 comprises ATM switching units configured in two stages and three columns.
The switching operation of the ATM cell in the above ATM switch is explained below, using a switching from input port HW.sub.01 to output port HW.sub.m0 as an example.
(1) A terminal unit (e.g. a telephone device), not shown in the drawing, is connected to input port HW.sub.01. Using a signaling cell, this unit requests the ATM switch to switch to output port HW.sub.m0. That is, (as described later) the signaling cell inputted from input port HW.sub.01 is inputted via a fixed path preset in a network within the switch to the signaling terminator 6.sub.0, and then to the switching controller 17.
(2) On receiving of the signaling cell, the switching controller 17 notifies the terminal unit connected to input port HW.sub.01 of a VCI for data transmission via a signaling terminator by using a signaling cell. It also outputs to VCC 10.sub.01 corresponding to input port HW.sub.01 switching data causing the incoming data cell added with the VCI to switch to output port HW.sub.m0.
(3) The earlier described terminal unit connected to input port HW.sub.01 makes cellular the data added with the VCI notified from the switch side and outputs them.
(4) VCC 10.sub.01 corresponding to input port HW.sub.01 adds to the cells outputted from the terminal unit the switching data corresponding to the VCI added to the cell.
(5) The network within the ATM switch, comprising ATM switching units 4.sub.00 through 4.sub.20 and 4.sub.01 through 4.sub.21, switches inputted cells in accordance with the switching data added to it, and data from the terminal unit connected to input port HW.sub.01 are outputted to output port HW.sub.m0.
FIG. 2 shows the sequence of ATM switching operations described above. In FIG. 2, protocols "SETUP", "CALL-PROC" (call processes), "ALERT", "CONN" (connecting), "CONN-ACK" (connection acknowledged), "DISC" (disconnecting), "REL" (releasing) and "REL-COM" (releasing completed) are all signaling data. Respective signaling data inserted in a plurality of signaling cells whose number corresponds to the data amount are transmitted and received. Since the concrete contents of the respective signaling data are not directly related to this invention, so their explanation is omitted.
For switching the signaling cells from input port HW.sub.01 to output port HW.sub.m0 in this switching cell, the following routes are conceivable.
(i) A route from ATM switching unit 4.sub.01 via ATM switching unit 4.sub.10 to ATM switching unit 4.sub.20.
(ii) A route from ATM switching unit 4.sub.01 via ATM switching unit 4.sub.11 to ATM switching unit 4.sub.20.
The following routes can also be considered for the part of the above route (i) between ATM switching unit 4.sub.01 and ATM switching unit 4.sub.10.
(a) A route from output port HW.sub.01 of ATM switching unit 4.sub.01 to input port HW.sub.m0 of ATM switching unit 4.sub.10.
(b) A route from output port HW.sub.11 (not shown in FIG. 1) of ATM switching unit 4.sub.01 to input port HW.sub.(m-1)0 (not shown in FIG. 1) of ATM switching unit 4.sub.10.
These switching routes can vary according to the hysteresis of the switching routes or the switching load (traffic volume) of the ATM switch.
When the terminal unit connected to input port HW.sub.01, in procedure (1) of the ATM cell switching operations in the ATM switch, requests a switching to output port HW.sub.m0 by using signaling cells, the outputted signaling cells added with particular VCIs by the terminal unit need to be introduced to the signaling terminator on the switch side.
As a method for setting a network of paths to a signaling terminator for realizing such operations, the method for setting a unity network of fixed paths among ATM switching units 4.sub.00 through 4.sub.21 at the initialization of the ATM switch is conventionally used. That is, VCC 10.sub.01 adds the switching data for specifying the fixed path as described above, as the switching data for its VCI, to inputted signal cells added with particular VCIs by the terminal unit connected to input port HW.sub.01.
Here, ordinarily, a plurality of signaling terminal units are provided, the number corresponding with the number of terminal units outputting signaling data. The example shown in FIG. 1 indicates the following. Signaling cells from terminal units corresponding to input ports HW.sub.00 through HW.sub.m0 are routed in a fixed manner to SIG 6.sub.0, which is connected to output port HW.sub.00 of ATM switching unit 4.sub.20. Signaling cells from the terminal units corresponding to input ports HW.sub.01 through HW.sub.m1 are routed in a fixed manner to SIG 6.sub.1, which is connected to output port HW.sub.m1 of ATM switching unit 4.sub.21.
As described earlier, a conventional ATM switch switches signaling cells to a signaling terminator, as in the line switching method.
Therefore, although signaling data can be properly switched to their corresponding signaling terminator when a switching state allowing fixed paths exists in a switching network, as described above, the workload (congestion) of the ATM switch increases and it becomes impossible to set the above-described fixed paths for inputted signaling cells when any of the ATM switching units on the above described fixed paths is already occupied by another ATM cell. In such a case, an ATM switch can no longer accept a call from a terminal unit, and the problem arises that the switching service is disrupted.
Furthermore, when a system comprises a plurality of signaling terminators, the above described fixed path setting method causes a load deviation at a particular signaling terminator according to the traffic volume. No control, such as for load sharing among the signaling terminators, has yet been realized.